1. Field of the Invention
The present invention relates to an oscillator using a dielectric resonator. More specifically, the present invention relates to a microwave oscillator using a dielectric resonator in a feedback loop of an amplifier.
2. Description of the Prior Art
An oscillator is provided by forming a positive feedback loop between, for example, a drain circuit and a source circuit, in an amplifying element such as a field effect transistor. Recently, an oscillator using a dielectric resonator as a feedback loop has been proposed and put into practical use by virtue of an excellent frequency stability. Such a microwave oscillator using a dielectric resonator is disclosed in, for example, U.S. Pat. No. 4,079,341 issued Mar. 14, 1978 to Donald Floyd Linn et al and entitled "MICROWAVE OSCILLATOR HAVING FEEDBACK COUPLED THROUGH A DIELECTRIC RESONATOR". Another disclosure of interest in this connection is also seen in IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, March, 1978, Vol. MTT-26, No. 3, entitled "A Highly Stabilized Low-Noise GaAs FET Integrated Oscillator with a Dielectric Resonator in the C Band". Such a microwave oscillators as disclosed in the above referenced U.S. patent and the above referenced paper employ a positive feedback loop implemented by a dielectric resonator coupled between the output and input of an amplifying element.
FIG. 1 is a view explaining a fundamental concept of a microwave oscillator employing a dielectric resonator as a positive feedback loop. Referring to FIG. 1, a dielectric resonator 11 is disposed within a casing 10 and a pair of exciting bars 12 and 13 are also housed in the casing 10 so as to be opposed to each other with the above described dielectric resonator 11 disposed therebetween. One exciting bar is coupled to, for example, a coaxial cable 14 through a connector, not shown. The coaxial cable 14 is connected to the input of an amplifier 15. The output of the amplifier 15 is connected by means of a coaxial cable 16 to a coaxial directional coupler or an RF monitor tee 17 as an example of a T type branch. A portion of the output from the amplifier 15 fed to the RF monitor tee 17 is withdrawn as a high frequency signal output of the oscillator. A portion of the output of the RF monitor tee 17 is also applied to a coaxial cable 18, which is connected to the other exciting bar 13 through a connector, not shown. With such an oscillator structure, a portion of the high frequency signal output of the amplifier 15 is fedback to the input of the amplifier 15 through the dielectric resonator 11, to thereby cause oscillation. The phase of the high frequency signal fedback to the input of the amplifier 15 is determined as a function of a phase difference between the exciting bars 12 and 13, the lengths of the coaxial cables 14, 16 and 18 and a phase difference between the input and output of the amplifier 15. The respective coupling coefficients between the exciting bar 12 and the dielectric resonator 11 and the exciting bar 13 and the dielectric resonator 11 are determined as a function of a gap g1 between the dielectric resonator 11 and the exciting bar 12 and a gap g2 between the dielectric resonator 11 and the exciting bar 13, respectively. Accordingly, in adjusting the phase by changing the amplification degree of the amplifier 15, the oscillation frequency and the like, it is necessary to change the lengths of the coaxial cables 14, 16 and 18 included in the feedback loop. Furthermore, in adjusting an attenuation degree of the feedback loop, it is necessary to change the gaps g1 and g2 between the exciting bars 12 and 13 and the dielectric resonator 11, which further necessitates a change in the size of the casing 10. In addition, since the dielectric resonator 11, the coaxial cables 14, 16 and 18 for phase adjustment and the amplifier 15 are implemented as separate blocks, a disadvantage results that the oscillator is large. Furthermore, a coaxial directional coupler, an RF monitor tee or the like 17 is required in order to withdraw a portion of the high frequency signal output of the amplifier 15 for feedback purposes. However, when the output signal for feedbacking is withdrawn by such RF monitor tee, another disadvantage results that a feedback gain is attenuated by approximately 10 to 20 dB, which necessitates an increased gain by the amplifier 15 accordingly.